Re: [PATCH 1/2] KVM: take kvm_lock for hardware_disable() during cpu hotplug

[Zachary Amsden <zamsden@redhat.com>]

On 11/17/2010 04:41 PM, Takuya Yoshikawa wrote:
> (2010/11/18 11:33), Zachary Amsden wrote:
>> On 11/17/2010 04:04 PM, Takuya Yoshikawa wrote:
>>> (2010/11/18 10:59), Zachary Amsden wrote:
>>>> On 11/15/2010 10:35 PM, Takuya Yoshikawa wrote:
>>>>> In kvm_cpu_hotplug(), only CPU_STARTING case is protected by 
>>>>> kvm_lock.
>>>>> This patch adds missing protection for CPU_DYING case.
>>>>>
>>>>> Signed-off-by: Takuya Yoshikawa<yoshikawa.takuya@oss.ntt.co.jp>
>>>>> ---
>>>>> virt/kvm/kvm_main.c | 2 ++
>>>>> 1 files changed, 2 insertions(+), 0 deletions(-)
>>>>>
>>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>>> index 339dd43..0fdd911 100644
>>>>> --- a/virt/kvm/kvm_main.c
>>>>> +++ b/virt/kvm/kvm_main.c
>>>>> @@ -2148,7 +2148,9 @@ static int kvm_cpu_hotplug(struct 
>>>>> notifier_block *notifier, unsigned long val,
>>>>> case CPU_DYING:
>>>>> printk(KERN_INFO "kvm: disabling virtualization on CPU%d\n",
>>>>> cpu);
>>>>> + spin_lock(&kvm_lock);
>>>>> hardware_disable(NULL);
>>>>> + spin_unlock(&kvm_lock);
>>>>> break;
>>>>> case CPU_STARTING:
>>>>> printk(KERN_INFO "kvm: enabling virtualization on CPU%d\n",
>>>>
>>>> I believe this is correct.
>>>
>>> You mean lock is not necessary?
>>
>> No, I believe your patch is correct and the lock should be there. Did 
>> you test with spinlock debugging just to be sure?
>>
>
> Sorry but no.
>
> I have no experience with cpu hotplug.
>
> So I thought it would take too much time to do real test by myself and 
> reported like this this time.
>
> Any easy way to test?

Yes, quite easy.  Some systems may not let cpu0 go offline, but you can 
manually disable and re-enable the other processors:

[root@mysore ~]# echo "0" > /sys/devices/system/cpu/cpu1/online
[root@mysore ~]# echo "1" > /sys/devices/system/cpu/cpu1/online

Cheers,

Zach